verse be the case, the solution under examination is evaporated to a known volume. This method is less acurate where high percentages are involved, as errors in observation will be multiplied. It is best to determine in this way from 1 to 2 per cent. of copper at the utmost.

In precipitating copper solutions containing iron, by means of ammonia, the ferric hydrate1 always retains some copper. This error is equalized by adding, in preparing the standard solutions, a quantity of iron corresponding about to the percentage of iron in the assay sample (assay of slag in Swansea). Organic substances, in presence of nitric acid, produce with ammonia a greenish tint, which exerts a disturbing effect in comparing the color of the copper solution (therefore, copper-schist should be ignited and filters incinerated before they are brought in contact with nitric acid). Ammoniacal solutions which may become turbid (for instance, if diluted with ordinary water containing lead) are allowed to become clear, and are filtered once more.

2. Jaquelin-Hubert's method for considerable percentages of copper.-Only one normal or standard solution of known strength is used. This is compared with the solution under examination in a graduated glass tube (calibrated). If richer than the standard, the assay solution is diluted to correspond with the former; and if poorer, the standard solution is correspondingly diluted, until an equal intensity of color has been obtained. From the relative volumes the percentage of copper is then readily calculated. The standard solution contains 0.5 gramme (7.71 grains) of copper in 1000 cubic centime ters (61.02 cubic inches). As errors of observation may easily occur, a gravimetric or volumetric assay is fre quently preferred for larger percentages of copper.

1 B. u. h. Ztg. 1869, p. 302.

III. SILVER.

30. PRINCIPAL ORES.

Native silver; amalgam, Ag and Hg, with 26.5 to 86Ag; antimonial silver, Ag and Sb, with 59 to 84 Ag; silver telluride (hessite), Ag Te, with 62.79 Ag; silver glance, Ag, S, with 87.1 Ag; brittle silver ore (stephanite), Ag, Sb S4, with 68.56 Ag; ruby silver (pyrargyrite), Ag, (Sb, As) S3, with 65.38 to 59.98 Ag; miargyrite, AgSbS, with 35.86 Ag; polybasite, Ag (Cu, Fe, Zn), Sb (As)S, with 64 to 75 Ag; stromeyerite, CuAgS, with 53 Ag; horn silver, AgCl, with 75.26 Ag; iodyrite (silver iodide) AgI, with 46 Ag; bromyrite (silver bromide) AgBr, with 57.45 Ag.

31. ASSAYS FOR NON-ALLOYS.1

The fire-assay methods are based upon the principle of decomposing the silver ore by means of lead or lead oxide, collecting the silver thus set free with an excess of the lead, the slagging off of foreign substances by suitable fluxes, and the cupellation of the lead button to separate the silver. The collection of silver in a lead button is effected, according to the nature of the foreign admixtures, either in a scorifier (scorification assay) or in a crucible (crucible assay).

The crucible assay permits of a large quantity of assay sample being used (which reduces the error from a loss of silver), and for this reait may be especially recommended for poor ores and such compounds as are free from antimony and arsenic (chloride, bromide, and

son,

1 Blossom, Gold and Silver Assays, in Am. Chemist, Jan. 1871, p. 250; Aaron, Pract. Treatise on Testing and Working Silver Ores, San Francisco, 1877.

is

iodide of silver and slags), and also for ores or sweepings of a very complex composition. The scorification assay is better adapted for ores containing sulphur, antimony, and arsenic, though likewise for other ores, so that this may be called an assay of general applicability; but, nevertheless, for the first-named ores, etc., the crucible assay simpler and cheaper and the result is more quickly attained. In America the crucible assay is chiefly used, while the scorification assay is preferred in Germany, although neither possesses any material advantage over the other.1

Wet assays are less commonly used.

I. Fire Assays.

A. Collecting the silver with lead.

1. Scorification assay.-This consists of an oxidizing fusion of the ores with lead, which becomes oxidized and yields up oxygen to the metallic sulphides, arsenides, and antimonides, forms a slag with the oxides thus formed and with the earths which may be present. The slagging off of basic earths is promoted by an addition of borax glass. The following points must be taken into consideration in preparing the charge.

a. The quantity of lead to be used will depend on whether the metallic sulphides, arsenides, and antimonides are easily or difficultly decomposed by lead oxide (p. 79), or whether they are entirely absent. Either granulated lead free from silver, which is measured (p. 31), is used, or argentiferous lead, whose percentage of silver is deducted from the assay by placing the silver button obtained from a corresponding quantity in the balance pan with the weights in weighing.

Lead sulphide is the easiest to decompose, next the sulphides of iron and zinc, and then copper sulphide. The most difficult are the sulphurized and arsenized nickel and cobalt ores.

1 B. u. h. Ztg. 1867, p. 102; 1874, p. 68; 1877, p. 232.

b. The quantity of borax depends on the degree of infusibility of the gangue (silicic acid and aluminous substances require but little, and lime and magnesia, much borax) and of the metallic oxides which are formed (ferric oxide, zinc oxide, stannic oxide, and nickel and cobalt ores require much, oxides of copper, bismuth, etc. but little).

In all cases, but little borax should be taken at first, to prevent the entire surface of the charge from being covered, as this would exclude the air. If more borax is necessary, it is added before the final heating. Much antimony and zinc oxide cause the cupels to become full of cracks.

c. The number of samples to be taken to control the accuracy of results will vary according to the richness of the ores, their want of uniformity, etc., from 2 to 10 or

more.

d. If the ore contains less than 1 per cent. of silver, 5 grammes (77.16 grains) or about A. T.' are taken for a charge, if more than 1 per cent., 2.5 grammes (38.58 grains) (from to A. T.), and of rich ores, 1 to very 0.5 gramme (15.43 to 7.71 grains) or about to A. T. In assaying very poor ores, the lead buttons obtained by scorification are concentrated by further scorification.

2

30 60

The following table gives some examples of various charges: